1. Field of the Invention
This invention relates to radio frequency systems using antenna arrays, and more specifically to the calibration of such systems.
2. Background
Antenna arrays may be used in any type of system that transmits or receives radio frequency signals using an antenna or antennas. Examples of such systems are radio communication systems, radars, and certain medical systems that employ radio frequency signals. The use of antenna arrays in such systems provides for antenna performance improvements over the use of a single element antenna. These antenna performance improvements include improved directionality, signal to noise ratio, and interference rejection for received signals, and improved directionality, security, and reduced transmit power requirements for transmitted signals. Antenna arrays may be used for signal reception only, for signal transmission only, or for both signal reception and transmission.
Typical antenna array systems consist of an array of antennas and a signal processor that combines the signals going to and coming from the individual array elements. This processing sometimes is called beamforming.
A typical application of antenna array systems is in a wireless communication system. Examples include a cellular communication system and a wireless local loop system. Such wireless communication systems consist of one or more communications stations, generally called base stations, each communicating with its subscriber units, also called remote terminals and handsets. In cellular systems, the remote terminal typically is mobile, while in a wireless local loop system the remote unit typically is in a fixed location. The antenna array typically is at the base station. Terminology for the direction of communication comes from conventional satellite communications, with the satellite replaced by the base station. Thus, communication from the remote terminal to the base station is called the uplink, and communications from the base station to the remote terminal is called the downlink. Thus, the antenna array transmits in the downlink direction and receives in the uplink direction. Antenna arrays also may be used in wireless communication systems to add spatial division multiple access (SDMA) capability, which is the ability to communicate with several users at the time over the same "conventional" (FDMA, TDMA or CDMA) channel. We have previously disclosed spatial processing with antenna arrays to increase the spectrum efficiency of SDMA and non-SDMA systems. See U.S. Pat. No. 5,515,378 issued 7 May 1996 entitled Spatial Division Multiple Access Wireless Communications System, incorporated herein by reference, U.S. Pat. No. 5,592,490 issued 7 Jan. 1997 entitled Spectrally Efficient High Capacity Wireless Communications Systems, also incorporated herein by reference, U.S. patent application Ser. No. 08/735,520 filed Oct. 23, 1996, entitled Spectrally Efficient High Capacity Wireless Communications Systems with Spatio-Temporal Processing, also incorporated herein by reference, and U.S. patent application Ser. No. 08/729,390 filed Oct. 11, 1996, entitled Method and Apparatus for Decision Directed Demodulation Using Antenna Arrays and Spatial Processing, also incorporated herein by reference. Systems that use antenna arrays to improve the efficiency of communications and/or to provide SDMA sometimes are called smart antenna systems. The above patents and patent applications are collectively referred to herein as "Our Smart Antenna Invention Documents."
With smart antenna communication systems, during uplink communications, one applies amplitude and phase adjustments to each of the signals received at the antenna array elements to select (i.e., preferentially receive) the signals of interest while minimizing any signals or noise not of interest--that is, the interference. Such amplitude and phase adjustment can be described by a complex valued weight, the receive weight, and the receive weights for all elements of the array can be described by a complex valued vector, the receive weight vector. Similarly, the downlink signal is processed by adjusting the amplitude and phase of the signals that are going to each of the antennas of the antenna array for transmission. Such amplitude and phase control can be described by a complex valued weight, the transmit weight, and the weights for all elements of the array by a complex valued vector, the transmit weight vector. In some systems, the receive (and/or transmit) weights include temporal processing, and in such cases, the receive (and/or transmit) weights may be functions of frequency and applied in the frequency domain or, equivalently, functions of time applied as convolution kernels.
Typically, the receive weight vector is determined from the spatial signature of a particular remote user, which in turn is determined by different techniques, for example from the uplink signals received at the antennas of the array from that remote user. The spatial signature (also called the receive manifold vector) characterizes how the base station array receives signals from a particular subscriber unit in the absence of any interference or other subscriber units. In normal operation, the receive weight vector may be determined by the spatial signature and any interference. The transmit weight vector used to communicate in the downlink with a particular user is also determined from the spatial signature of the particular user. It is thus desirable to determine the transmit weight vector from the receive weight vector for a particular user.
Time division duplex (TDD) systems are those in which uplink and downlink communications with a particular remote user occur at the same frequency but different time slots. Frequency division duplex (FDD) systems are those in which uplink and downlink communications with a particular remote user occur at the different frequencies.
Practical problems may make determining the transmit weight vector from the receive weight vector for a particular user difficult to do. Time division duplex (TDD) systems are those in which uplink and downlink communications with a particular remote user occur at the same frequency but different time slots. Frequency division duplex (FDD) systems are those in which uplink and downlink communications with a particular remote user occur at the different frequencies. Because of the well known principle of reciprocity, it might be expected that determining the transmit weight vector from the receive weight vector might be straightforward. However, in the uplink, the received signals that are being processed may be somewhat distorted by the receive apparatus chains associated with each of the antenna elements of the antenna array. The receive apparatus chain includes the antenna element, cables, filters, RF electronics, physical connections, and analog-to-digital converter ("ADC") if processing is digital. In the case of a multi-element antenna array, there typically is a separate receive apparatus chain for each antenna array element, and thus the amplitude and the phase each of the received signals at each element may be distorted differently by each of the receive apparatus chains. A receive weight vector that does not take this into account will be in error, causing less than optimal reception at the base station. However, in practice, communications will still be possible. When one transmits downlink signals through the antenna array, each of the signals radiated by an antenna element goes through a different transmit apparatus chain, thus possibly causing different amplitude and phase shifts in the transmitted signals. If the transmit weight vector was derived from a receive weight vector that did not take the differences in the receive apparatus chains into account, transmission from the base station may be hard to achieve. Further difficulty may result if the transmit weight vector does not take differences in the transmit apparatus chains into account, possibly making communication using such a transmit weight vector not optimal. The purpose of calibration is to determine calibration factors for compensating for the different amplitude and phase errors that occur in the signals in the receive chain and the different amplitude and phase errors that occur in the transmit chain. It should be added that the phase and amplitude shifts that occur in the receive and transmit apparatus chains are, in general, frequency dependent.
Determining the transmit weight vectors from the receive weight vectors for a particular user is more difficult in the case of an FDD system because reciprocity may no longer be assumed. One needs to additionally take into account the differences in propagation in the uplink and downlink. Once one does take such differences into account, there still is a need to determine calibration factors for compensating for the different amplitude and phase errors that occur in the signals in the receive chain and the different amplitude and phase errors that occur in the transmit chain.
Originally, manufacturers of antenna array systems used signal processors that assumed ideal antenna arrays in which all transmit and receive electronics were assumed to be perfect, or in which the transmit and receive apparatus chains were assumed to be identical for each of the antennas. As a result, these antenna array systems were not only difficult to design and manufacture, they were prohibitively expensive and subject to errors, interference, and drifts over time. Using receive weights for determining transmit weights may not lead to effective communications with such a system.
It is known that compensation can be achieved by convolving each of the m signals received or transmitted by the antenna elements by a complex calibration function (i.e., by a complex valued time sequence), where each calibration function describes the transfer function correction required to compensate for the gain and phase errors a signal undergoes when passing through the transmit (or receive) apparatus chains. In some systems, this can be simplified to multiplicative correction, where each calibration function is a calibration factor--a complex valued number that describes the required amplitude and phase correction required for compensation. In general, the set of calibration functions are the elements of a calibration vector function, one complex valued calibration vector function for the transmit path, and one for the receive path, where each function is a time sequence. In the case of multiplicative correction, the set of calibration factors are the elements of a calibration vector, one complex valued calibration vector for the transmit path, and one for the receive path. Prior-art methods for determining array calibration vector functions involve measurements that have several associated drawbacks. Firstly, the methods require external measuring equipment which may be expensive, unwieldy and cumbersome to use repeatedly. Secondly, conventional calibration methods are sensitive to drifts in system parameters, such as frequency references, over the extended period of time during which measurements are being taken, and these drifts result in inaccuracies in the measured array calibration vector. In addition, some prior art techniques only determine multiplicative rather than convolution kernel calibration factors, and there are frequency dependent components in the antenna array. In order to eliminate this frequency dependence and still use multiplicative calibration vectors, it is necessary to calibrate the antenna array for each frequency channel of communication. Thirdly, the transfer characteristics of the RF electronics depend on changing ambient conditions such as temperature, humidity, etc., which makes it essential that antenna arrays be repeatedly calibrated in their ambient environment.
Harrison et al. disclose in U.S. Pat. No. 5,274,844 (Dec. 28, 1993) a method for calibrating transmit and receive calibration vectors (as complex valued vector transfer functions) in two experiments which involve a data bus connecting a resource controller to a remote terminal. In the first experiment, the data bus indicates to the remote terminal to send a known signal to the base station. This determines the receive apparatus chain calibration. In a second experiment, the signals received at the remote terminal are sent back to the resource controller via the data bus to enable determining the transmit apparatus chain calibration.
Co-owned U.S. Pat. No. 5,546,090, issued 13 Aug. 1996, and assigned to the assignee of the present invention, discloses a calibration method which can determine both transmit and receive calibration vectors using a simple transponder co-located with the remote terminal that re-transmits to the base station the signals received at the remote terminal from the base station. Such a method does not require the wired data-bus of the Harrison et al. invention. Still, additional transponder equipment is required.
While these prior art calibration methods provide separate calibrations for the receive and transmit paths, and also calibrate for the different air paths between the base station antenna elements and the subscriber unit, the methods require special calibration apparatus.
PCT Patent application publication WO 95/34103 (published Dec. 14, 1995) entitled Antenna array calibration, Johannisson, et al., inventors, discloses a method and apparatus for calibrating the transmission (and reception) of an antenna array. For transmit calibration, an input transmit signal is inputted into each antenna element one antenna at a time. After the input transmit signal has passed through a respective power amplifier, the signal transmitted by each antenna element is sampled by a calibration network. The resulting signal is fed into a receiver, and a computation means relates the received signal with the original transmit signal for each antenna element. Correction factors can then be formed for each antenna element. The antenna elements may then be adjusted (in amplitude and phase, or in-phase I and quadrature Q components) using the correction factors so as to ensure that each element is properly calibrated during transmission. For receive calibration, an known input signal is generated and injected using a calibration network (a passive distribution network) into each antenna element of the antenna array. The signals pass from the antenna elements trough respective low noise amplifiers, and the signals thus received by each antenna element are measured by a beam forming apparatus. The beam forming apparatus can then generate correction factors by comparing the injected signal with the measured signals so as to individually calibrate each antenna element. The correction can be described as amplitude and phase corrections, or as corrections in in-phase I and quadrature Q components.
While the Johannisson et al. method provides separate calibrations for the receive and transmit paths, the method requires special calibration apparatus.
Thus there is a need in the art for a calibration method and apparatus which is simple, both in terms of the equipment necessary and the time required, so that calibration can be performed repeatedly and rapidly wherever and whenever desired. There also is a need in the art for a simple calibration technique that only uses existing base station electronics and does not require special calibration hardware. There also is a need in the art for a method that enables one to determine transmit weight vectors from receive weight vectors, including calibrating for the receive apparatus and transmit apparatus chains, the calibration obtained using simple techniques that use existing base station electronics and do not require special calibration hardware.